A fuel cell is a galvanic cell that generates electrical energy by converting chemical energy, derived from a fuel supplied to the cell, directly into electrical energy by an electrochemical process in which the fuel is oxidized in the cell. A typical fuel cell includes an anode, a cathode, electrocatalysts and an electrolyte housed in a casing. The fuel material and oxidant are continuously and independently supplied to the anodes and cathodes, respectively, where the fuel and oxidant react chemically to generate a useable electric current. The reaction by-products are withdrawn from the cell.
A great advantage of a fuel cell is that it converts chemical energy directly to electrical energy without the necessity of undergoing any intermediate steps, for example, combustion of a hydrocarbon or carbonaceous fuel as takes place in a thermal power station. A fuel cell reactor may comprise a single-cell, or a multi-cell stack. In either case, the membrane/electrode assembly (MEA), comprising the proton-conducting membrane (the electrolyte) and the anode and cathode, is typically sandwiched between two highly (electrically) conductive flow field plates that may serve multiple functions. First, these plates may function as current collectors providing electrical continuity between the fuel cell voltage terminals and electrodes. Additionally, the flow field plates provide mechanical support for the MEA and distribute the reactants and water across the active area of the MEA electrodes, which is accomplished by a flow field imprinted into the side of each plate in direct contact with the electrodes of the MEA. It is well known that the performance of a fuel cell is highly dependent on the efficient transport of reactants to the electrodes, on the uniform humidification of the MEA, and on the appropriate water management of the cell, i.e., the supply and removal of water produced during operation of the cell. Since flow field design controls the reactant concentration gradient, flow rate, pressure drop and water distributions, the flow field design affects the performance of fuel cells.
Tie rods and end plates hold the fuel cell assembly together. Feed manifolds are respectively provided to feed the fuel (such as hydrogen, reformed methanol or natural gas) to the anode and the oxidant (air or oxygen) to the cathode via the fluid flow field plates. Exhaust manifolds are provided to exhaust excess fuel and oxidant gases and water and other by-products formed at the cathode.
Multi-cell structures comprise two or more such fuel cell assemblies connected together in series or in parallel to increase the overall power output of the fuel cell as required. In such arrangements, the cells are typically connected in series, wherein one side of a given plate is the anode plate for one cell, and the other side of the plate is the cathode plate for the adjacent cell and so on.
The flow field is imprinted into the side of each flow field plate in direct contact with the electrodes of the MEA. The flow field provides distribution/flow channels to distribute the reactants across the active area of the MEA electrodes and remove by-product and water.
The performance of the fuel cell is highly dependent on the efficient transport of reactants to the electrodes, on the removal of by-products and water away from the electrodes, and on the appropriate fluid management of the cell. Flow field design affects the performance of an electrochemical fuel cell because flow field design controls the reactant concentration gradient, distribution, flow rate, pressure drop and water/by-product removal.
Recently, several problems have been recognized in the art with respect to flow field design and the reactant flow channel configurations, especially in fuel cells that use liquid fuels such as methanol as reactants. Key problems with these prior art designs include inadequate fuel flow distribution, high pressure-drops across the MEA and poor removal of by-products and water.
Conventional flow field designs typically comprise either pin or serpentine designs. An example of a flow field design of the pin-type is illustrated in U.S. Pat. No. 4,769,297 in which an anode flow field plate and a cathode flow field plate have each projections, which may be referred to as pins. The fuel flows across the anode plate through the intervening grooves formed by the projections, with the oxidant similarly flowing through intervening grooves formed in the cathode flow field plate. Other examples of flow fields having a pin-type design are illustrated in U.S. Pat. No. 4,826,742. Pin-design flow fields result in low reactant pressure drop across the corresponding flow field, however, reactants flowing through such flow fields tend to follow the path of least resistance across the flow field that may result in channeling and the formation of stagnant areas. This in turn results in poor fuel cell performance.
An example of a flow field incorporating a single serpentine design is illustrated in U.S. Pat. No. 4,988,583. As shown in FIG. 2 of U.S. Pat. No. 4,988,583, a single continuous fluid flow channel is formed in a major surface of the flow field plate. A reactant enters through the fluid inlet of the serpentine flow channel and exits through the fluid outlet after traveling over a major part the plate. Such a single serpentine flow field forces the reactant flow to traverse the entire active area of the corresponding electrode, thereby eliminating areas of stagnant flow. However, this channeling of reactant across the active area results in a relatively high reactant flow path length that creates a substantial pressure drop and significant concentration gradients from inlet to outlet. Additionally, the use of a single channel to collect the entire liquid water product from the electrode may promote flooding of the single serpentine, especially at high current densities.
U.S. Pat. No. 4,988,583 also tries to address this pressure drop problem by providing an embodiment in which there are several continuous separate flow channels. The multiple serpentine flow field design is illustrated in FIG. 4 of U.S. Pat. No. 5,108,849.
The flow field designs described have certain drawbacks, especially in fuel cells using methanol as the reactant fuel. In such systems, the by-products are large quantities of carbon dioxide gas and water. The main drawbacks include:                Unfavorable pressure drop across the flow field. Long, narrow flow paths, as found in the serpentine designs, lead to high pressure drops inside the flow channels. In these cases, high parasitic power is required to pressurize the reactants.        Stagnant areas within the flow field. The reaction rate of the fuel cell is normally slower in stagnant areas, hence significantly affecting the performance of the fuel cell. The formation and presence of stagnant areas leads to ineffective utilization of the electrode catalyst. This occurs when the reactants are in the liquid form and the by-products are gases with limited solubility in the liquid reactants under operating conditions. The by-products form gas bubbles that may adsorb on the electrode surface and cover the active catalyst area and/or upset/hinder the flow of reactants in the flow field.        Flooding of the electrode. In cases where the reactants are in the gas phase while the by-products are liquid, poor removal and accumulation of the by-products inside the cell will promote flooding. Flooding reduces the efficiency of the fuel cell since less reactant is exposed to the catalyst in the electrode.        High concentration gradients of reactants across the flow field. Long flow paths may cause significant concentration gradients from the inlet to the outlet of the fuel cell, and lead to non-uniform current distribution through the fuel cell.        
These problems and others are addressed by the flow field designs of the present invention.